Answer: Every time you are foliar feeding & when ever the pH changes out of acceptable range on the reservoir (usually not more than a half a point, example" 6.0 - 6.5, if it went to 6.7 you would want to lower it a little) One should remember to check and adjust pH "AFTER" adding the nutrients, additives and or wetting agents as most nutrients and additives will change the pH. You are wasting your time adjusting the pH before hand. Also adjust pH "AFTER" mixing up foliar spray and before spraying plants.